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From page 156... ...
have argued that practical quantum computing is fundamentally impossible, the committee did not find any fundamental reason that such a system could not be built  provided the current understanding of quantum physics is accurate. Yet significant work remains, and many open questions need to be tackled to achieve the goal of building a scalable quantum computer, both at the foundational research and device engineering levels.

From page 157... ...
While researchers have successfully engineered individual qubits with high fidelities, it has been much more challenging to achieve this for all qubits in a large device. The average error rate of qubits in today's larger devices would need to be reduced by a factor of 10 to 100 before a computation could be robust enough to support error correction at scale, and at this error rate, the number of physical qubits that these devices hold would need to increase by at least a factor of 105 in order to create a useful number of effective logical qubits.

From page 158... ...
However, it is also possible that even with steady progress in QC R&D, the first commercially useful application of a quantum computer will require a very large number of physical qubits  orders of magnitude larger than currently demonstrated or expected in the near term. In this case, government or other organizations with long time horizons can continue to fund this area, but this funding is less likely to grow rapidly, leading to a Moore's lawtype of development curve.

From page 159... ...
Avoiding this scenario requires some funding to continue even if commercial interest wanes. Key Finding 2: If nearterm quantum computers are not commercially successful, government funding may be essential to prevent a significant decline in quantum computing research and development.

From page 160... ...
The results of this work will have a profound impact on the rate of development of largescale quantum computers and on the size and robustness of a commercial market for quantum computers. Even in the case where nearterm quantum computers have sufficient economic impact to bootstrap a virtuous cycle of investment, there are many steps between a machine with hundreds of physical qubits and a largescale, errorcorrected quantum computer, and these steps will likely require significant time and effort.

From page 161... ...
For quantum computing, an obvious metric to track is the number of physical qubits operating in a system. Since creating a scalable quantum computer that can implement Shor's algorithm requires improvements by many orders of magnitude in both qubit error rates and number of physical qubits, reaching this number in any reasonable time period requires a collective ability of the R&D community to improve qubit quantity per device exponentially over time.

From page 162... ...
comes into play once QC technology has improved to the point where early quantum computers can run errorcorrecting codes and improve the fidelity of qubit operations. At this point, it makes sense to start tracking the effective number of logical qubits6 on a given machine, and the time needed to 4 Other metrics have been proposed  and new metrics may be proposed in the future  but most are based on these parameters.

From page 163... ...
from the measured error rates using different numbers of physical qubits. For concatenated codes, this comes from the number of levels of concatenation needed, and for surface codes, it is the size (distance)

From page 164... ...
Tracking the number of logical qubits has clear advantages over tracking the number of physical qubits in predicting timing of future errorcorrected quantum computers. This metric assumes the construction of errorcorrected logical qubits with a target gate error rate, and naturally reflects progress resulting from improvements in the physical qubit quality or QEC schemes which decrease the physical qubit overhead and lead to more logical qubits for a given number of physical qubits.

From page 165... ...
More data points  and, more importantly for Metric 1, consistent reporting on the effective error rate using RBM on onequbit and twoqubit gates within a device  would make it easier to examine these trends and compare devices. For the rest of this chapter, machine milestones mapping progress in QC will be measured in the number of doublings in qubit number, or halving of the error rate required from the current stateoftheart functioning QC system, which is assumed to be the order of 24 physical qubits in mid2018, with 5 percent error rates.

From page 166... ...
quantum computers versus year; note the logarithmic scaling of the vertical axis. Data for trapped ions are shown as squares and for superconducting machines are shown as circles.

From page 167... ...
that has been used to compare classical computer performance for many decades. This 8 These applications could include different quantum errorcorrecting codes, variational eigensolvers, and "classic" quantum algorithms, and should be able to run on differentsize "data sets" to enable then to be able to measure differentsize quantum computers.

From page 168... ...
was originally a simple set of commonly used programs and has changed over time to more accurately represent the compute loads of current applications. Given the modest computing ability of nearterm quantum computers, it seems clear that at first these applications would be relatively simple, containing a set of common primitive routines, including quantum error correction, which can be scaled for differentsize machines.

From page 169... ...
The first benchmark machine is the class of digital (gatebased) quantum computers containing around 24 qubits with average gate error rate better than 5 percent, which first became available in 2017.

From page 170... ...
However, this is necessarily a moving target, as improvements continue to be made in the approaches for classical computers that the quantum computers are trying to outperform. For a rough estimate of the limit of a classical computer, researchers have benchmarked the size of the largest quantum computer that a classical computer can simulate.

From page 171... ...
FEASIBILITY AND TIME FRAMES OF QUANTUM COMPUTING 171 FIGURE 7.4 An illustration of potential milestones of progress in quantum computing. The arrangement of milestones corresponds to the order in which the committee thinks they are likely to be achieved; however, it is possible that some will not be achieved, or that they will not be achieved in the order indicated.

From page 172... ...
Achieving quantum supremacy requires a task which is difficult to perform on a classical computer but easy to compute on the quantum data plane. Since there is no need for this task to be useful, the number of possible tasks is quite large.

From page 173... ...
While both trapped ion and superconducting qubits have demonstrated qubit gate error rates below the threshold required for error correction, these gate errorrate performances have not yet been demonstrated in systems with tens of qubits, nor are these early machines able to measure individual qubits in the middle of a computation. Thus, creation of a machine that successfully runs QEC, yielding one or more logical qubits of better error rates than possible with physical qubits, is an important milestone.

From page 174... ...
While achieving quality improvement through QEC shows that building a logical qubit is possible, the overhead of QEC is strongly dependent on the error rates of the physical system, as shown earlier in Figure 7.1. Improvement in both areas is required in order to achieve an errorcorrected quantum computer that can scale to thousands of logical qubits.

From page 175... ...
There are a large number of system issues that would need to be solved before these largescale machines could be realized. First, owing FIGURE 7.5 Schematic of a modular design approach to a largescale, fault tolerant quantum computer.

From page 176... ...
7.3.7 Milestone Summary The time to create a large faulttolerant quantum computer that can run Shor's algorithm to break RSA 2048, run advanced quantum chemistry computations, or carry out other practical applications likely is more than a decade away. These machines require roughly 16 doublings of the number of physical qubits, and 9 halvings of qubit error rates.

From page 177... ...
The community expects quantum supremacy • Decrease average error rate to better than 0.5% (10× these machines to exist by the early better than G1 machines)

From page 178... ...
A3/G3  Commercially useful • Identify useful task that a NISQ computer can Funding of QC will likely be impacted if quantum computer carry out more efficiently than a classical computer. this milestone is not available in mid to • Hone the corresponding quantum algorithm for late 2020s.

From page 179... ...
7.4.1 The Global Research Landscape Publicly funded U.S. R&D efforts in quantum information science and technology are largely comprised of basic research programs and proofofconcept demonstrations of engineered quantum devices.12 Recent initiatives launched by the National Science Foundation (NSF)

From page 180... ...
In general, they span a range of subfields, and are not focused on quantum computing exclusively. As of the time of this writing, the United States had released a National Strategic Overview for Quantum Information Science, emphasizing a sciencefirst approach to R&D, building a future workforce, deepening engagement with industry, providing critical infrastructure, maintaining national security and economic growth, and advancing international cooperation [22]

From page 181... ...
over quantum computation, Computation seven years silicon quantum and computation, and Communication quantum resources and Technology integration Sweden Wallenberg 2017 SEK 1 billion Quantum Center for (US$110 computers, quantum Quantum million) simulators, quantum Technology communication, quantum sensors; sponsored by industry and private foundation China National 2017 76 billion Centralized quantum Laboratory Yuan research facility for Quantum (US$11.4 Information billion)

From page 182... ...
It is likely that observations and experiments on the performance of quantum computers throughout the course of QC R&D will help to elucidate the profound underpinnings of quantum theory and feed back into development and refinement of quantum theory writ large, potentially leading to unexpected discoveries. More fundamentally, development of elements of the theories of quantum information and quantum computation have already begun to affect other areas of physics.

From page 183... ...
.14 Last, progress in quantum computing can be a unique source of motivation for classical algorithm researchers; discovery of efficient quantum algorithms has spurred the development of new classical approaches that are even more efficient and would not otherwise have been pursued [3235] .15 Fundamental research in quantum computing is thus expected to continue to spur progress and inform strategies in classical computing, such as for assessing the safety of cryptosystems, elucidating the boundaries of physical computation, or advancing methods for computational science.

From page 184... ...
The same types of qubits currently being explored for applications in quantum computing are being used to build precision clocks, magnetometers, and inertial sensors  applications that are likely to be achievable in the near term. Quantum communication, important both for intra and intermodule communication in a quantum computer, is also a vibrant research field of its own; recent advances include entanglement distribution between remote qubit nodes mediated by photons, some over macroscopic distances for fundamental scientific tests, and others for establishing quantum connections between multiple quantum computers.

From page 185... ...
Thus, building and maintaining strong QC research groups is essential for this goal. Key Finding 8: While the United States has historically played a leading role in developing quantum technologies, quantum information science and technology is now a global field.

From page 186... ...
. There are a number of emerging quantum software development platforms pursuing an open source environment.18 Support for open quantum computing R&D has helped to build a community and ecosystem of collaborators worldwide, the results and advances of which can build upon each other.

From page 187... ...
7.5 TARGETING A SUCCESSFUL FUTURE Quantum computing provides an exciting potential future, but to make this future happen, a number of challenges will need to be addressed. This section looks at the most important ramifications of the potential ability to create a large faulttolerant quantum computer and will end with a list of the key challenges to achieve this goal.

From page 188... ...
However, significant work remains before a quantum computer with practical utility can be built. In the committee's assessment, the key technical advances needed are: • Decreased qubit error rates to better than 10–3 in manyqubit sys tems to enable QEC.

From page 189... ...
Kalai, 2011, "How Quantum Computers Fail: Quantum Codes, Correlations in Physical Systems, and Noise Accumulation," preprint arXiv:1106.0485.

From page 190... ...
Vanian, 2017, "IBM Adds JPMorgan Chase, Barclays, Samsung to Quantum Comput ing Project," Fortune, http://fortune.com/2017/12/14/ibmjpmorganchasebarclays othersquantumcomputing/; J Nicas, 2017, "How Google's Quantum Computer Could Change the World," Wall Street Journal, https://www.wsj.com/articles/howgooglesquantumcomputer couldchangetheworld1508158847; Z

From page 191... ...
Jones, 2017, "20Qubit IBM Q Quantum Computer Could Double Its Predecessor's Processing Power," Digital Trends, https://www.digitaltrends.com/ computing/ibmq20qubitsquantumcomputing/; S.K. Moore, 2017, "Intel Accelerates Its Quantum Computing Efforts With 17Qubit Chip," IEEE Spectrum, https://spectrum.ieee.org/techtalk/computing/hardware/ intelacceleratesitsquantumcomputingeffortswith17qubitchip.

From page 192... ...
Goldstone, and S Gutmann, 2014, "A Quantum Approximate Optimi zation Algorithm Applied to a Bounded Occurrence Constraint Problem," preprint arXiv:1412.6062.

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